Movements of the earth's crust that lead to earthquakes and related seismic phenomena are of great interest to earth scientists who seek to scientifically understand these occurrences and to government entities, as well as to ordinary citizens who seek to minimize the lethal potential of earth movements and even to predict quakes. One only has to contemplate the loss of property and life caused by earthquakes around the world and, particularly, in Southern California, to realize the significance of earthquake studies.
Today it is generally accepted that much seismic activity is due to motion of huge tectonic plates that make up the upper regions of the earth's crust. These plates either slip laterally past one another or actually collide with resulting uplift of mountains and the subduction of one plate under another. This process of plate movement naturally places great strains on the crustal material. When the strain becomes too great, the material gives way, slippage occurs along a fault, and an earthquake results.
Even before the movement of plates was generally understood, scientists realized that earthquakes involved movement and displacement of the earth surface along fault lines. For example, the famous 1906 San Francisco earthquake, which occurred along California's San Andreas fault, resulted in a lateral displacement of several feet which can be readily observed as deviations in roads and fences crossing the fault in Marin County, just north of San Francisco. As a result of this and similar observations, scientists attempted to measure movement along faults to understand and even predict quakes.
Some faults show horizontal movement, while others show vertical movement or a mixture of horizontal and vertical movement. The initial fault measurements involved the use of traditional surveying techniques. Using optical sighting with a theodolite, a network of measurements is made parallel to and across a fault line. In that way even small movements of the fault can be detected. Although the sighting instruments can be left in place permanently, the usual procedure is to have each location marked with a permanent marker (geological survey monument) and to periodically set up portable instruments to make the required measurements. The normal movement along faults is often in the range of a only a few millimeters to a few centimeters per year. However, during an actual earthquake much larger movements, often of several meters, occur. After an earthquake has occurred, it is necessary to rapidly survey the entire fault system to determine what type of movement has occurred and precisely where it has occurred. There is often a premium in being able to rapidly and accurately make the required measurements because understanding the movement caused during the quake may help pinpoint risks from aftershocks.
Although most fault measurements made today are still aimed at determining the distances between monument sites, the technologies employed in the measurements have been changing. In many cases simple sighting through a theodolite has been replaced by newer, more accurate techniques. One great improvement is the use of automated sighting instruments that employ a laser beam to ensure alignment of the sites. A limitation with any form of optical sighting is that the monuments must fall on a line of sight. That means that measurements from mountaintops to valley bottoms may be difficult if intervening ridges or trees or human development blocks the line of sight. Also, meteorological conditions such as rain or fog can temporarily render sighting impossible. Therefore, there has been a series of attempts to use nonoptical measuring methods.
It is possible to measure the distance between two sites by measuring the time it takes a radio wave to travel from one site to another. However, to detect a change in distance of only a few centimeters requires bulky and expensive instruments. It is not economically feasible to make a large number of monument measurements using such a system. In recent years the U.S. Department of Defense has been developing a satellite-based system (the Navigation Satellite Timing and Ranging [NAVISTAR] Global Positioning System [GPS]). The GPS system should make highly accurate measurements, such as those required along earthquake faults, relatively simple and economical.
The basic idea behind the GPS system is to have to a number (at least 18) of special satellites orbiting the earth in stable and well-known orbits. The key to the system is that the satellites each contain an extremely accurate atomic clock. The clock time is coded into a high-frequency radio signal transmitted by each satellite. If a ground-based receiver has a similarly accurate clock, the time for the signal to travel from the satellite to the ground can be determined. Thus, the distance from the satellite to the ground receiver can be found.
A major source of inaccuracy in such a measurement is refraction of the radio wave by the ionosphere. This results in a measurement that does not reflect the shortest distance between the satellite and the ground receiver. This problem is largely eliminated by having the satellites transmit on two different frequencies. Because the different frequencies are refracted differently by the ionosphere, it is possible to make corrections based on the two frequencies that yield the true distance. By measuring the distance from more than one satellite to the ground receiver trigonometry can be used to find the exact location of the receiver in both horizontal and vertical terms. This is ideal for geodetic earthquake measurements, as faults cause both horizontal and vertical displacements. The accuracy of the measurements depend on the type and quality of the receiver, but currently-available portable receivers can yield measurements accurate within a few millimeters or so. This is definitely in the range required for the fault line measurements.
The GPS receivers can be readily adapted to fault line measurements by mounting the receiver's antenna 40 on a typical surveyor's tripod 10 directly above the center of the monument whose position is to be measured. Because the measurement reflects the vertical position of the antenna, it is important to employ a device that allows ready control of the antenna elevation. A prior art method for employing portable GPS receivers attaches the antenna 40 to a top platform 12 of a typical surveyor's tripod 10. As seen in FIGS. 1, the portable GPS antenna 40 comprises sections of different-sized metallic cylinders arranged concentrically. The antenna 40 bears a threaded socket (not shown) by which it can be threaded onto a tribrac adaptor 96 (see FIG. 1).
As seen in FIG. 2, the typical tripod 10 is topped by a substantially flat platform 12 with a large central aperture 14. The tribrac adaptor 96 comprises an anodized aluminum disc which engages the tribrac 95. FIG. 1 shows the tribrac 95, which has a precision optical system by which an operator can look through a side-mounted eyepiece 97 and determine whether the tribrac 95 is located directly over the monument center. The entire tripod setup must be carefully moved from side to side until the tribrac optics indicate that exact placement has been achieved. Then a series of leveling screws 98 are adjusted to ensure that the top surface of the tribrac is perfectly level horizontally. This ensures that the antenna holder 99 which is part of the tribrac adaptor 96 is plumb.
If the limited range of the leveling screws 98 is insufficient to level the device, legs 16 of the tripod 10 must be adjusted. Because the monuments are frequently located in rugged terrain, considerable jockeying of the tripod 10 may be necessary to achieve a level and on mark tribrac 95. Finally, when the tribrac 95 is level and exactly over the center of the monument, the height of the antenna 40 must be determined. Because the tripod legs 16 must be adjusted to achieve correct leveling, the height of the entire structure varies from setup to setup. The exact height of the antenna 40 is determined by using a tape measure to measure the distance between the center of the monument and the lower edge of the largest antenna circle. The exact height of the antenna 40 above the monument is then calculated trigonometrically.
Needless to say, the entire process is very time consuming. Furthermore, the process is susceptible to numerous errors that may compromise the accuracy of the GPS measurement. The measurement of antenna height is particularly fraught with error, because tape measures are notoriously difficult to employ accurately. Furthermore, an entire series of data points must be adjusted for different antenna heights at each measurement point. Mathematically this is trivial, but entering all the height data constitutes yet another potential error.
Unfortunately, the optical system of the tribrac 95 is extremely delicate. A sharp blow, or even vibrations, may decalibrate the optics. If the optics are out of calibration, the actual placement of the antenna 40 may be off by several millimeters--a distance that may be as large as the measured fault movement. This problem can be discovered at the end of a series of field measurements by testing optical calibration of the tribrac, but there is no way to know when the tribrac 95 went bad so the entire series of measurements must be discarded, thereby ruining days of work.